The invention relates to a threading device for guiding the path of a tail separated from a full-width web along a portion of a paper machine or a paper finishing device, and a method for threading the tail of the web. Below in this description, the tail of the web can also be referred to with the shorter expression xe2x80x9ctailxe2x80x9d.
For guiding the web along a portion of the paper machine, in other words between two or more sections in the paper machine, such as the drying section and the calender section, a narrow strip, the so-called tail, is separated from the full-width web, to be transferred along the portion before guiding the full-width web to the portion. The purpose of this is to secure the controlled, faultless run of the tail and further the whole web, wherein first after the tail has passed the portion without problems, a substantially wider part to follow the tail is separated from the web by cutting and is further widened to guide the full-width web following the tail to the portion.
Before threading, the web usually runs freely down e.g. to a pulper or the like placed underneath the last drying cylinder in the drying section, wherein a relatively heavy portion of the web, extending several meters, can be in a state uncontrolled as such, which further complicates the controlled threading. Problems occur particularly at the stage of starting to move the tail from the uncontrolled, downwards running movement to run in a controlled manner e.g. between cylinders in a calender.
Known solutions for guiding the tail are primarily based on the use of air jets in connection with various conveyor troughs and guide plates. The use of air jets as such easily develops a phenomenon that when the tail meets a nip between rolls in the next section it has a fold in the transverse direction and/or it is in an otherwise random position in its transverse direction, which may cause wrinkling of the tail, thereby slowing down the progressive movement of the tail. This phenomenon makes it difficult to guide the tail into the nip and between the cylinders, and may even cause damage in parts of the paper machine. In several known solutions, this is due to the fact that breaking off of the tail cannot be influenced to a sufficient degree, but the breaking off takes place at random as such and often first after a considerable portion of the tail has already been moved into the nip or between cylinders or the like.
In threading of the paper web through calenders, it has been common to use rope threading systems in which the tail is guided between threading ropes arranged at the ends of rolls. At the same time, the tail must be pulled aside from the actual running path of the paper web.
The rope systems involve known drawbacks, such as the space required by them in the cross direction of the machine and the said pulling to the side, which may be a problem with weak paper grades. Furthermore, high web running speeds impose requirements on the functional reliability of the rope system.
Finnish patent 52478 discloses a solution for cutting the tail before moving the tail to the portion of the paper machine. The solution is based on a chopper arranged to operate in a way that it chops the web into pieces before it ends up in a pulper or the like, whereby the tail is left intact only on the length which is needed for guiding the tail by using air jets or the like. In this way, it is possible to provide the web with a short and light-weight portion of the tail connected thereto, which can be moved to the run in a controlled manner. This solution also prevents the tail from being double-folded when it enters a nip or the interspace between cylinders. However, the apparatus according to this solution is complex, and its blades increase occupational safety risks.
Finnish patent 62695 discloses a solution based on planar guide plates. The guide plates are arranged at a certain angle to each other in a way that the direction of the first plate corresponds to the direction of guiding the cut tail end further, and the direction of the second plate corresponds to the direction of guiding the tail into a pulper or the like. The guide plates are arranged to co-operate with air jets provided in connection with the same, to act as guide and conveying surfaces so that when the air jets draw the tail into opposite directions, the tail is cut by breaking between the plates, and the cut end of the tail is guided by the action of the air jets into the nip. The end running into the pulper is blown into the vicinity of the guide plates and the air jets by a separate air jet. A drawback in the described solution is the relatively large number of air jets, the great need of pressurized air, and the noise caused by the jets, as well as the unnecessary and disturbing air flows produced in their vicinity. The solution is reliable as such, but for example external air flows may cause disturbances in it. Furthermore, the tail must be bent onto a wide curve during the guiding, whereby it may be folded or turned away from its correct position.
Furthermore, the use of air guides for guiding the tail on the length of a certain portion from the drying section to a rope gap formed by threading ropes is disclosed e.g. in Finnish patent 89288 and in German application publication 3924897. However, all of the solutions described above have the drawback that, particularly in the beginning of the run, the tail is, in spite of cutting, still in a relatively uncontrolled state. The above-mentioned problems related to guiding of the tail to the run cannot thus be fully eliminated.
As a mechanical solution, a suction transfer mat or a suction belt is used, which is a continuous loop-like structure arranged to run through two rolls or the like located at a distance from each other. The purpose of the tail is to run along with the upper surface of a revolving mat, for which purpose the upper surface of the mat is provided with a suction through the mat. The use of the suction transfer mat usually requires several successive suction transfer mat units to span one portion of the paper machine, each unit requiring a separate power arrangement with an electric motor or the like driving system with associated power transmission, and further at least one suction device and mat-specific arrangements for directing a vacuum onto the surface of the mat. The tail is usually brought onto the first suction transfer mat with an air jet, and the transfer of the tail further from one mat to another requires separate arrangements, whereby the structure as a whole becomes rather complex and expensive and it does not very easily provide a continuous, fully controlled tail threading. Such arrangements are presented e.g. in European patent 232689 and in Swedish published specification 420431.
It is the purpose of the invention to eliminate the above-mentioned drawbacks and to improve the prior art by presenting a threading device and a method for threading the tail of the web, wherein the tail can be conveyed over the whole portion of the paper machine in a controlled manner and wherein even very long threading lengths can be accomplished. For achieving this aim, the threading device according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1. The device comprises a suction channel provided with reduced pressure for gripping the tail, movable in the portion of the paper machine, and transfer means for transferring the suction channel in the portion of the paper machine and also for releasing the tail along the portion. The method for threading the tail of the web is characterized in what will be presented in the characterizing part of the independent claim 17. In the method, the inlet opening of the suction channel provided with reduced air pressure is first moved close to the tail, after which the tail is moved into the suction channel by means of the reduced pressure prevailing therein and the tail is released along the portion by moving the suction channel.
Transfer tubes are prior known in connection with a paper machine only as fixed systems for removing extra edge strips to be cut off the web. These solutions, which are not related to threading and the problems involved therein, are presented e.g. in Finnish patent 62155 and in German published specification 1253570.
The threading device according to the invention has several advantages, the most essential to be mentioned being that the tail can, in a controlled manner, be taken to be transferred by the device, directly at the beginning of a portion of the paper machine or paper finishing device, for example immediately after the last drying cylinder in the drying section. Thus, the tail is at no stage left exposed to the effect of e.g. external air flows or machine parts. In a preferred embodiment, the device comprises also a cutting means for cutting the tail before starting its transfer to the portion, wherein a transverse fold in the tail is avoided. The device requires considerably smaller air flows than in the known solutions, which contributes to the reliability of the device and reduces e.g. noise problems. Furthermore, it is obvious that the relative simplicity of the structure makes the device advantageous also in view of e.g. maintenance operations. As a considerable advantage, it should also be mentioned that the device and the method can be applied in a flexible manner at different web running speeds and for different grades and strengths of paper to be manufactured.
By means of the invention, the tail of the web can be placed in a controlled manner onto the path determined by the web geometry even at high web running speeds and also in complex web running geometries.
Other features characteristic to the threading device and to the method for threading the tail of the web according to the invention will be disclosed in the appended dependent claims.